Related fields include semiconductor devices and their fabrication; in particular, thin-film components of resistive-switching non-volatile memory (ReRAM).
Nonvolatile memory elements are used in computers and other devices requiring persistent data storage (e.g., cameras, music players). Some traditional nonvolatile memory technologies (e.g., EEPROM, NAND flash) have proven difficult to scale down to smaller or higher-density configurations. Therefore, a need has developed for alternative nonvolatile memory technologies that can be scaled down successfully in terms of performance, reliability, and cost.
In resistive-switching-based nonvolatile memory, each individual cell includes a bistable variable resistor. It can be put into either of two states (low-resistance or high-resistance), and will stay in that state until receiving the type of input that changes it to the other state (a “write signal”). The resistive state of the variable resistor corresponds to a bit value (e.g., the low-resistance state may represent logic “1” and the high-resistance state may represent logic “0”). The cell is thus written to by applying a write signal that causes the variable resistor to change resistance. The cell is read by measuring its resistance in a way that does not change it. Preferably, write and read operations should require as little power as possible, both to conserve energy and to avoid generating waste heat.
Many ReRAM devices change resistance by creating and destroying, or lengthening and shortening, one or more conductive paths through a variable-resistance layer or stack while the bulk material remains static (e.g., it does not change phase). The bulk material is often a highly insulating dielectric. The conductive paths (also known as “percolation paths”) are formed when an electric field organizes conductive or charged defects or impurities into a filament stretching from one interface to the other, with sufficient defect density that charge carriers can easily traverse the layer by tunneling from defect to defect. To return the variable resistor to the high-resistive state, it is often not necessary to destroy the entire filament, but only to introduce a gap too wide for tunneling somewhere along the filament's length. Some of the types of defects that have been used include metal clusters and oxygen (or nitrogen) vacancies.
The “forming process” that creates the very first filament in a newly fabricated ReRAM cell is risky. The defects may be randomly scattered through the bulk of the variable-resistance layer, or they may be in some other layer such as an electrode or other source layer. Some cells may need stronger electric fields than others to collect the defects into a filament because the initial defect distributions may vary from cell to cell. Substantial force is necessary to push impurity atoms through a solid (e.g., metal, oxygen, or nitrogen) or break ionic bonds typical of high-ionicity materials such as hafnium oxide (HfOx) and other “high-k” dielectrics. The risk is of “over-forming;” creating a filament so wide or dense that the operating write signals are too weak to break it. An over-formed cell cannot be rewritten. At this point the entire device has been built, so the investment has been significant and the cost of failure is high.
One approach to protecting ReRAM variable-resistance layers from over-forming is the addition of a current-limiting element, such as a transistor or resistor, to each cell. Transistors are relatively complex and there is a limit to how far they can be scaled down to provide increased memory density. Resistors—referred to as “embedded resistors” when they are built into the cell—may in some cases add excessive thickness so that patterning the layers into cells becomes a challenge, particularly for 3D arrays.
Therefore, a need exists for a way to prevent overcurrent through ReRAM variable-resistance layers that is simple, that can be scaled down with the rest of the cell to facilitate increased memory density, and that adds only a tolerable amount of extra thickness to the stack.